CO2 emerging uses
While CO2 is already utilized in many products and solutions, global emissions are far in excess of the amounts used in these.
Significant research and innovation efforts are being directed towards utilizing CO2 in many other applications and in the production of diverse products - from chemicals, to fuels to even food.
Most of these efforts are in their early stages of research or commercialization, but the such efforts could result in a future in which CO2 has been transformed from a problem to an opportunity.
Liquid fuels consist of hydrocarbon chains, and are conventionally derived from the refining of crude oil.
CO2 can also be converted to equivalent fuels. In order to achieve this, CO2 needs to react with a source of hydrogen under appropriate conditions.
The current pathways for converting CO2 into liquid hydrocarbon fuels can take one of the following routes:
A direct route in which CO2 is reacted with hydrogen or water under specific temperatures and pressures, and with the aid of special catalysts, producing hydrocarbon fuels. In this pathway, conversion of CO2+H2O to fuels might however be more challenging than conversion of CO2+H2 to fuels. Note however that the H2 used in the latter reaction could itself have been derived from water through electrolysis.
Another promising route is the indirect route in which CO2 is first reacted with water to first form synthesis gas (also called syngas), which is a mixture of CO and H2. Syngas can then be converted into a range of hydrocarbon fuels through well developed thermo-chemical pathways.
Conversion of CO2 to chemicals can use pathways that are used for conversion to hydrocarbon fuels - the fuels themselves are a class of chemicals.
For instance, methanol, one of the main chemical building blocks, can be derived from syngas (synthesis gas, CO + H2). In fact, this is how methanol is currently being produced from fossil sources.
In this process, instead of producing syngas from fossil sources as is done currently, syngas can be produced from a reaction of CO2 and water. This pathway thus ensures that the methanol and other chemicals derived from it are made from CO2 emissions.
As in the case of fuels, CO2 can also be alternatively converted into methanol and other chemicals through a direct hydrogenation of CO2 (CO2 + H2) with the use of select catalysts.
CO2 is a thermodynamically inert molecule with a high bond dissociation energy of 750 kJ/mol. Due to such high stability of its bond, it does not on its own react with water or hydrogen at standard atmospheric conditions to produce chemicals or fuels. It is hence necessary to set up suitable environments for reactions to take place and an external force in the form of catalysts to accelerate these reactions.
CO2 can be converted to different chemicals and fuels using the gas-phase reaction, liquid-phase reaction, photocatalytic reaction, or electrochemical reaction. The gas-phase reaction includes the dry reforming of methane using CO2 and CH4, or CO2 hydrogenation using CO2 and H2. The liquid-phase reaction includes formic acid formation from pressurized CO2 and H2 in aqueous solution. The photocatalytic reaction is commonly known as artificial photo-synthesis, and produces chemicals from CO2 and H2O under light irradiation. The electrochemical reaction can produce chemicals from CO2 and H2O using electricity.
Different catalysts work well for different types of conversion phases (gas-phase, liquid phase, etc.) and for different target products even for specific types of reaction. For electrochemical conversion for instance, it has been shown that specific metals can produce specific products, such as Au or Ag for CO, Sn or Bi for formate, Cu for C2H4 etc.
Thus, depending on the desired end product and the conversion process employed, different catalysts could do be the preferred choices. Even for specific reactions and end products, multiple catalysts might present themselves as suitable candidates, though only one of two of them might produce the highest performances. For instance, while catalysts comprising different metals like Cu, Zn, Ag, Cr, and Pd have been employed for CO2 hydrogenation to methanol, copper-based catalysts have shown some of the highest efficiencies.
Some of the prominent classes of catalysts for CO2 conversion to fuels and chemicals are:
Copper, on its own or in combination (for instance, copper/zinc oxide for methanol synthesis) is a popular catalyst used for many CO2 conversion reactions.
Among carbon-based materials, graphitic carbon nitride (g-C3N4) is an inexpensive and sustainable catalyst that is being used for CO2 conversion to chemicals and fuels.
Nickel catalysts are also used for some of the conversions.
CO2 conversion chemicals can also use the biological pathway, in which they typically use microbes to do the job.
Depending on the type of microorganism, they may either consume CO2 to produce energy (similar to plants) or exhale CO2 after energy production (similar to humans).
Microalgae are the most prominent microorganisms that consume CO2 and give out oxygen in return like humans and animals. Many microalgae species have the potential to convert CO2 into specific fuels, chemicals and other valuable products. Given that there are over 30,000 different strains (and counting) of microalgae, they hold significant promise in the realm of CO2 to chemicals and fuels.
Most other microbes - bacteria, fungi, yeast - consume sugars, and not CO2, for energy production. However, in recent efforts, scientists have been able to engineer the metabolism of these microbes such that they consume CO2 and produce valuable chemicals.
Carbon dioxide dissolves readily in water and this enables it to have many natural reactions with the environment.
CO2 to limestone on land
Carbon dioxide is removed from the atmosphere by dissolving in water and forming carbonic acid. The reaction is as follows: CO2 + H2O -> H2CO3 (carbonic acid).
Over a period of time, carbonic acid weathers rocks, and forms, among others, limestone. This is a prominent example of a land-based natural sequestration of CO2.
CO2 to limestone in oceans
In fact, in the early stages of earth's formation, as the oceans formed, carbon dioxide that was present in high concentrations in the atmosphere dissolved to form soluble carbonate compounds. This was one of the reasons for the reduction of atmospheric CO2 in the atmosphere, which in turn enabled many forms of life.
From the bicarbonate compounds thus formed, along with calcium present in the sea water, calcium carbonate is formed. This is precipitated and utilized by marine organisms like coral to build reefs. The reaction is as follows:
Ca++ + 2HCO3- -> CaCO3 + CO2 + H2O
The carbon in this case gets sequestered on the seafloor as limestone.
Calcium carbonate comprises more than 4% of the earth’s crust and is found throughout the world. Its most common natural forms are chalk, limestone, and marble, produced by the sedimentation of the shells of small fossilized snails, shellfish, and coral over millions of years.
Calcium carbonate is a useful and versatile material. The main applications of calcium carbonate are in building materials, ceramic tiles, blackboard chalk, iron ore purification, oil well drilling fluids, paints, adhesives, and sealants. Here are some specifics on the wide range of its applications:
It is widely used in the paper, plastics, paints and coatings industries both as a filler – and due to its special white colour - as a coating pigment.
It is used as an effective dietary calcium supplement, antacid, phosphate binder, or base material for medicinal tablets. It also is found on many grocery store shelves in products such as baking powder, toothpaste, dry-mix dessert mixes, dough, and wine.
It is the active ingredient in agricultural lime, and is used in animal feed. Calcium carbonate also benefits the environment in these sectors through water and waste treatment.
It is critical to the construction industry, both as a building material in its own right (e.g. marble), and as an ingredient of cement. It contributes to the making of mortar used in bonding bricks, concrete blocks, stones, roofing shingles, rubber compounds, and tiles.
Lime (CaO), an important material in making steel, glass, and paper, is derived from calcium carbonate.
Because of its antacid properties, calcium carbonate is used in industrial settings to neutralize acidic conditions in both soil and water.
Diamonds are pure carbon - 99.95% by weight.
It can be easily seen the connection between CO2 and a diamond - the “C”.
All the same, it is not such a straightforward process to make diamonds from CO2. While lab grown diamonds are already established, they use graphite as the starting point, and not CO2.
Some companies have recently started producing diamonds from CO2, with Aether being the prominent among them. Aether’s process involves first capturing the CO2 from an emissions source, converting it into methane in the second step and using the methane to produce pure carbon in the form of diamond in the third. The company follows the chemical vapour deposition method to make diamonds. In this method, methane (derived from CO2 in Aether's case) is passed through a plasma of hydrogen gas, depositing the carbon on a surface, and slowly forming a diamond in the process. In a plasma the atoms are excited, and this lets the nuclei and electrons move freely. The conditions in the plasma allow the methane to break up, and for its carbon to be deposited in the form of diamond without any need for the enormous pressures that are needed for diamonds to form in nature.
While the upstarts are trying to capture CO2 in diamonds, the old boys are trying to capture the CO2 into the diamond mines. De Beers for instance has formed a new program called “CarbonVault,” which will explore use of rocks in the diamond mines it owns around Kimberley to store CO2, through the use of different pathways to enhance the rock forming process.
By the way, the original diamonds inside the earth likely had a CO2 origin too. The Big Hole next to the Kimberly Diamond Mine in South Africa was explored by geologists who inferred, based on their investigations, that the CO2 escaping from the volcanic eruptions underground of the site was absorbed into the area’s peculiar rocks and developed into diamonds over millions of years.